Require C++20

[bdice]

Description

This updates the RMM library target and basic example to require C++20.

Closes #2034

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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📝 Walkthrough

Walkthrough

This PR updates build, formatting, and test helper code to use C++20-oriented configuration and formatting.

Changes

C++20 Standard Upgrade

Layer / File(s)	Summary
C++20 standard requirement updates cpp/CMakeLists.txt, cpp/examples/basic/CMakeLists.txt, .clang-format	rmm sets CXX_STANDARD to 20, basic_example requires cxx_std_20, and .clang-format uses Standard: c++20.
DeviceScalarTest signature formatting cpp/tests/device_scalar_tests.cpp	DeviceScalarTest::random_value() overload signatures and requires constraints are reformatted without changing the underlying logic.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Possibly related PRs

• rapidsai/rmm#2413 — Touches cpp/tests/device_scalar_tests.cpp with related C++20-style requires and signature updates.

Suggested reviewers: wence-, shrshi

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and directly summarizes the main change to require C++20.
Description check	✅ Passed	The description accurately states that RMM and the basic example now require C++20.
Linked Issues check	✅ Passed	The PR updates the RMM target and basic example to C++20, satisfying the linked requirement.
Out of Scope Changes check	✅ Passed	The remaining edits are C++20-related configuration and formatting changes, with no unrelated functionality added.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

cpp/CMakeLists.txt (1)

133-141: ⚠️ Potential issue | 🟠 Major | ⚡ Quick win

Propagate the C++20 requirement to downstream consumers.

The rmm target sets CXX_STANDARD 20 for its own compilation, but this property does not propagate to consumers linking against rmm::rmm. To ensure downstream projects inherit the C++20 requirement, add target_compile_features(rmm PUBLIC cxx_std_20) after the set_target_properties() call.

Suggested patch

 set_target_properties(
   rmm
   PROPERTIES BUILD_RPATH "\$ORIGIN"
              INSTALL_RPATH "\$ORIGIN"
              CXX_STANDARD 20
              CXX_STANDARD_REQUIRED ON
              CXX_VISIBILITY_PRESET hidden
              POSITION_INDEPENDENT_CODE ON
              INTERFACE_POSITION_INDEPENDENT_CODE ON)
+
+target_compile_features(rmm PUBLIC cxx_std_20)

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@cpp/CMakeLists.txt` around lines 133 - 141, The rmm target sets its own
CXX_STANDARD but doesn't propagate C++20 to consumers; add a call to
target_compile_features for the rmm target to export the requirement (i.e., call
target_compile_features(rmm PUBLIC cxx_std_20)) immediately after the
set_target_properties(...) block so downstream targets linking to rmm::rmm
inherit the C++20 requirement.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Outside diff comments:
In `@cpp/CMakeLists.txt`:
- Around line 133-141: The rmm target sets its own CXX_STANDARD but doesn't
propagate C++20 to consumers; add a call to target_compile_features for the rmm
target to export the requirement (i.e., call target_compile_features(rmm PUBLIC
cxx_std_20)) immediately after the set_target_properties(...) block so
downstream targets linking to rmm::rmm inherit the C++20 requirement.

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📥 Commits

Reviewing files that changed from the base of the PR and between f0f416a and 045de9b.

📒 Files selected for processing (2)

• cpp/CMakeLists.txt
• cpp/examples/basic/CMakeLists.txt

[lamarrr]

LGTM, thanks for working on this!

[bdice]

/merge